The present invention relates to an optical fiber characteristic evaluation system for evaluating the transmission characteristics of many optical fibers forming optical transmission lines in a communication system or the like, and a computer-readable recording medium on which a computer program used for the system is recorded.
Recently, optical fibers have been widely used as signal transmission lines in communication systems and the like in place of conventional metal lines.
To lay and manage such optical fibers, a test on an optical fiber transmission line and evaluation of the test result must be periodically performed.
Assume that one optical fiber includes a plurality of optical fibers connected in series.
Conventionally, optical fibers are tested by using an optical time-domain reflectometer (OTDR) like the one shown in FIG. 10.
In this OTDR, an optical pulse from an optical pulse generator 10 is input to one end of one of optical fibers 2.sub.1 to 2.sub.n of an optical multi-fiber cable 1 through a directional coupler 11 and an optical path switch 12, and a photodetector 13 detects the intensity of light (backward scattered light or reflected light) returning to the one end side of the optical fiber. An A/D converter 14 then converts the photodetection signal output from the photodetector 13 into a data signal, and inputs it to a signal processor 15. The signal processor 15 processes a series of data signals until a predetermined period of time elapses after the incident timing of the optical pulse, and stores the distance versus received light intensity characteristic data in a characteristic data memory 16.
After the characteristic data of the optical fiber forming one transmission line are acquired, the optical path switch 12 is switched to connect another optical fiber to the directional coupler 11, thus storing the characteristic data of the optical fibers 2.sub.1 to 2.sub.n in the characteristic data memory 16.
The transmission characteristics of each optical fiber are evaluated as follows. The characteristic data stored in the characteristic data memory 16 is selectively output to an image display unit or printer (not shown) to display a waveform on the coordinate plane defined by the distance axis and the received light intensity axis, as shown in FIG. 11. Variations in attenuation value and the presence/absence of a disconnection or the like are checked from the slope of the displayed waveform, local level changes, and the like.
To detect the overall characteristics of the optical multi-fiber cable 1 as a bundle of optical fibers 2.sub.1 to 2.sub.n or evaluate the characteristics of one optical fiber in contrast to the characteristics of the overall transmission line, instead of independently evaluating each of the optical fibers 2.sub.1 to 2.sub.n all the characteristic data stored in the characteristic data memory 16 must be displayed on the same coordinate plane and evaluated.
In tests on optical fibers using the above OTDR, the characteristic data obtained from optical fibers having the same characteristics vary due to variations in the wavelength of an optical pulse and variations in connection loss in the optical path switch 12.
If, therefore, the waveforms of the characteristic data obtained by this OTDR are displayed on the same coordinate plane, the displayed waveforms greatly shift from each other in the vertical direction than the variations of the characteristic data, even though the optical fibers have almost the same characteristics. For this reason, it is very difficult to detect the overall characteristics of the cable and evaluate the characteristics of one optical fiber in contrast to the characteristics of the overall transmission line.
Generally, one transmission line is made by connecting a plurality of optical fibers each having a length of several km.
In this case, amounts of connection losses between fibers greatly vary due to variations in connection points and transmission lines.
Since the variations in connection losses are accumulated in the transmission line having a plurality of connection points, if using optical fibers having the same characteristics, the characteristics of the overall transmission line differ from every transmission lines.
In evaluating a characteristic of a certain connection point in a certain transmission line in the overall transmission line, the characteristic data of the near portion of the certain connection point greatly varies due to accumulating variations in the connection loss and accumulated variations in connection loss in the optical path switch 12.
For this reason it is very difficult to evaluate the characteristic of the certain connection point only.
In recent years, with an increase in the number of optical fibers of the optical fiber cable 1, the OTDR can acquire the characteristics of several ten to several thousand optical fibers. This makes it more difficult to perform evaluation.